Heat-resistant black fibers and fabrics derived from rayon



United States Patent 3,484,183 MAT-RESISTANT BLACK FIBERS AND FABRICS DERIVED FROM RAYON Arthur D. Dickson, Roseville Village, Edward M. Peters, Grant Township, Washington County, Minu., assiguors to Minnesota Mining and Manufacturing Company, St. Paul, Minn, a corporation of Delaware No Drawing. Filed June 4, 1965, Ser. No. 461,533

Int. Cl. D06m l/18 US. Cl. 8--116 20 Claims ABSTRACT OF THE DISCLOSURE Heat-resistant black fibers and fabrics are produced by impregnation with a metal phosphate and thermochemical transformation of corresponding rayon fibers and fabrics which retain fiber identity but are changed to a flexible black state having a different chemical composition and radically different chemical and physical properties. Both insulating and electrically conductive black fibers and fabrics are disclosed.

This invention relates to new and useful man-made organic fibers, and fabrics thereof, which have good flexibility and tensile strength and that are per se black, inert, thermally stable, flame-resistant and resistant to intense heat. They have good thermal electrical insulating properties. They can be carbonized without loss of fiber identity to provide corresponding fibrous material which is electrically conductive.

These novel fibrous materials are produced by a thermochemical transformation of corresponding rayon (regenerated cellulose) fabrics and fibers, which retain fiber identity but are changed to a flexible black state having a different chemical composition and radically different chemical and physical properties. In this process the rayon precursor, impregnated with a water-soluble monobasic metal phosphate salt, is heated for a short time to a final temperature of at least 450 F., in the presence of an oxygen-containing atmosphere, as more fully disclosed hereafter.

A woven rayon cloth can be transformed to this black state without being rendered fragile or brittle and may have a yarn tensile strength (breaking strength) at least 20% as great as that of the original rayon cloth. It has substantially the same physical structure and appearance except for its lustrous jet black color. The fabric has good abrasion resistance and it does not blacken the fingers when stroked.

The original polymeric cellulose molecules of the rayon fibers are not only changed by pyrolysis but by accompanying chemical compounding reaction whereby from about 3% up to about 8% by weight of phosphorous atoms (apparently carbon-bonded) and from about 3% up to about 8% by weight of alkali or alkaline earth metal atoms (both being supplied by the metal phosphate salt), become chemically combined in the stable polymeric oxygen-containing carbon-compound molecules of the insulative black product fibers. This insulative fiber product is non-carbonized, is not charred, is free from elemental carbon, and it is non-tarry. The carbon content as determined by analysis is in the range of about 50 to 65%. Other elements can also be introduced. Boron atoms can be incorporated in the polymer structure by using a mixture of the metal phosphate salt and boric acid to impregnate the rayon starting material. The thermochemical transformation is rapid, requiring not over about 30 minutes at oven temperatures which are in the range of about 450 to 900 F.; and a total oven exposure of about 10 minutes or less can be employed under suitable conditions.

3,484,183 Patented Dec. 16, 1969 The lustrous jet black opaque appearance of the fabrics and fibers is due to substantially complete absorption of all visible Wave-lengths of incident light by fibers which are smooth-surfaced and translucent (permitting incident light to penetrate and be absorbed in the body of the fiber).

This process provides Woven and nonwoven fabrics, yarns, staple fibers, and continuous filaments, of useful strength and flexibility, which are flame-resistant and are resistant to high temperatures, and which are good electrical and heat insulators. They do not become fragile upon prolonged or repeated exposure in air to temperatures as high as 600 F., and even higher, and can tolerate short exposures to temperatures higher than 1000 F. without becoming fragile. A blowtorch flame does not cause combustion even when applied until the fabric has finally disintegrated and vaporized. These black fibrous materials have a high degree of chemical inertness; they are highly resistant to sulfuric acid and to alkalies; and the essential properties are retained when subjected to prolonged or repeated boiling in water (which leaches out any soluble residues present).

This product can be rapidly carbonized without loss of fiber identity to make useful electrically conductive flexible black fiber materials of 70 to 99+% carbon content as described more fully in a later section. This is accomplished by heating for a short time in a nonoxidizing environment to a final temperature in the range of about 950 to 4700 F. (about 500 to 2600 C.). Surprisingly, carbonizing in an oxygen-free nitrogen atmosphere can result in nitrogen fixation so as to introduce chemically bonded nitrogen atoms into the carbonaceous fibers.

A prior copending application of one of us (that of E. M. Peters, Ser. No. 426,789, filed Jan. 21, 1965 as a continuation-in-part of application filed Apr. 14, 1960), since issued as Patent No. 3,235,323 (Feb. 15, 1966), described a somewhat similar process of making black insulative fibrous products from corresponding rayon starting material, except that the rayon was impregnated with a suitable nitrogenous salt; this product containing about 3 to 8% by weight of combined nitrogen atoms (apparently carbon-bonded). A mixture of nitrogenous salt and boric acid could be used to obtain desirable boron-containing products. Use of dibasic ammonium phosphate to impregnate the rayon resulted in a black insulative product which, after washing, contained about 2% by weight of phosphorus in addition to the aforesaid combined nitrogen. The insulative nitrogen-containing fibrous products which contained about 50 to carbon, could be carbonized by heating for a short time at a high temperature in a nonoxidizing environment to produce electrically conductive nitrogen-containing fibrous material containing to 99+% carbon.

In that work it had been assumed, prior to the present discovery, that use of a nitrogenous salt as rayon impregnaut was essential, since it was believed that the rapid formation of strong flexible insulative black fibers required introduction and the presence of carbon-bonded nitrogen atoms and that these were needed to obtain good thermal stability and to permit of rapid carbonization capable of providing conductive carbonaceous fibers of good flexibility and strength.

Contrary to such assumptions, we have unexpectedly discovered that certain non-nitrogenous metal phosphate salts can be employed (even in the absence of any nitrogeneous compound) to impregnate the fibrous rayon starting material to obtain, upon suitable heating for a short time at a temeprature of at least 450 F., useful insulative heat-resistant black fibers and fabrics which contain both phosphorus atoms and metal atoms derived from the salt. Furthermore, we have discovered that these non-nitrogenous fiber materials can be carbonized 3 at higher temperatures to provide useful fiber products which are electrically conductive and contain 70 to 99+ carbon.

Specifically, we have found four monobasic (primary) metal salts of orthophosphoric acid that are useful for this purpose. These are: monomagnesium phosphate,

monocalcium phosphate, Ca(H PO monosodium phosphate, NaH PO and monopotassium phosphate, KH PO These salts are also known by the names: magnesium dihydrogen phosphate or magnesium biphosphate, calcium dihydrogen phosphate or calcium biphosphate, sodium dihydrogen phosphate or sodium biphosphate, potassium dihydrogen phosphate or potassium biphosphate, respectively. They all contain the dihydrogen phosphate group (H PO and are slightly acidic in reaction. They are moderately water soluble (to the extent of at least by weight at room temperature).

It is possible to employ a mixture of such non-nitrogenous metal phosphate salt and a nitrogenous salt of the type described in the aforesaid copending Peters application, to thereby obtain black insulative product fibers that do contain nitrogen atoms as well as phosphorus atoms (in addition to metal atoms). In this case a different route is afforded than is taught in that application (which describes use of a non-metal ammonium phosphate salt -to provide product fibers containing both nitrogen and phosphorus). The present metal phosphate salts, moreover, make possible the production of black fibers having a higher content of combined phosphorus than is possible by the process of that application; as well as introducing magnesium, calcium, sodium or potassium atoms into the fiber molecules (or a combination thereof when a mixture of two or more of the metal salts is used).

Use of the magnesium or calcium salt impregnant results in a nearly complete loss of strength followed by a partial regain of breaking strength to a useful value in converting a woven rayon fabric to a corresponding useful flexible black organic fabric that is heat resistant and insulative, containing 50-65% carbon, by suitable heating at a temperature of at least 450 F. Thus when the impregnated rayon cloth is heated in air at 550 F., the yarn breaking strength drops to less than 10% of the initial value during the first two minutes of exposure and then rises to over 25% of the initial value during the next five minutes, following which the strength remains substantially constant for at least twenty minutes of further heating air and then gradually decreases. This increase in strength appears to coincide with the inclusion of the carbon-bonded phosphorus atoms and metal atoms in the pyrolized fiber molecules which are being transformed to a new state. This differs from what is obtainable when plain untreated rayon fibers are heated. (Comparable heating in air of an untreated rayon cloth at 550 F. resulted in a steady loss of strength to a value at the end of 100 minutes which was only 10% of the original value.)

In contrast, when the sodium or potassium salt is used as the impregnant, under these same heating conditions, there is no sudden loss and subsequent regain of strength. The yarn breaking strength decreases during the first two minutes of oven exposure to a value about 20% as great as the initial value and then levels off, to result in a useful flexible black fabric which contains combined phosphorus and sodium or potassium.

Despite this difference in behavior during making, and despite the differences in the divalent and monovalent metal atoms that are embodied in the black fibers of the fabric product, it appears that these fabrics can be used interchangeably for most heat-resisting insulative uses, and they can each be carbonized to yield useful conductive fabrics.

The greatest product strength and flexibility are obtained when boric acid (or equivalent boron compo n s cmployed in admixture with the metal phosphate salt to impregnate the rayon starting material. This results in at least 0.5% by weight of boron atoms being introduced into the molecular structure of the black fibers, and boron is retained even when these fibers are carbonized at high temperatures. Thus, rayon yarns impregnated with monosodium phosphate (NaH PO and boric acid in 5 to 1 ratio by weight, which were heated to 450 to 700 F. in an oxygen-containing atomsphere and were then immediately carbonized by heating in pure nitrogen up to 2600 F. (the total heating exposure being approximately 5 minutes) yielded flexible black conductive yarns containing 82% carbon and having tenacity values in the range of 2.6 to 3.0 grams per denier. These latter products also contained approximately 2.5% boron, 4.4% nitrogen, 0.8% hydrogen, a trace of sodium, and the balance was phosphorus and oxygen.

The unique properties of the insulative black fibrous products (50 to carbon) suggest various practical uses. For instance: Heat-resistant insulative articles of wearing apparel, aprons and gloves for metallurgical and foundry workers and for fire fighters. Fire barrier drapes. Also, fire-resistant insulating sleeves and wrapping tapes for electrical conductors and cables and for piping and conduits. Also, fire-resistant fibrous insulating bats. Fibers in flock form-can be used for providing heat-resistant flocked coatings. Fabrics, yarns and continuous filaments can be used to advantage in fiber-reinforced plastics subject to high temperatures; including ablation types. Also as backings or reinforcing fibers in high-temperature types of insulating tapes and adhesive tapes, including pressuresensitive adhesive tapes. Also, inert acid-resistant filter cloths for filtering very hot gases, liquids and even molten metals. The present yarns have better heat resistance than industrial asbestos yarns; and the fabrics have been thermal insulating properties than asbestos fabrics.

The following described process is employed in making the insulative black fiber products of this invention from rayon textile fiber starting materials, which can be in the form of cloth (woven, knitted or felted); nonwoven fibrous bats, felts, fabrics or tissues formed of staple fibers; yarns, rovings, continuous filament tows, etc., whose structural identity can be retained throughout the process. (The term rayon, as here used, is employed in the modern sense as designating manmade regenerated-cellulose filaments and the textile fibers, yarns and fabrics made therefrom. Such filaments and fibers may range in size from about 5 to about 30 microns in diameter. The term does not include cellulose acetate fibers and fabrics; but it does include high tenacity regenerated cellulose fibers made by saponifying (hydrolyzing) oriented cellulose acetate fibers, as illustrated by Fortisan cellulose fibers, yarns and fabrics.) An important economic feature is that these rayon fibers and fabrics are readily and inexpensively available from the textile industry, and that the process is relatively simple, rapid and economical and needs no unusual or elaborate equipment. Thus the woven cloth products can be manufactured directly and inexpensively from woven rayon cloths of substantially the same physical structure. Cotton and other natural cellulosic fiber materials are not equivalent and cannot be employed.

The rayon starting material in continuous web, yarn, roving or tow form can be processed in continuous fashion from start to finish, which is preferable in factory production although batch procedures can be used when making up small lots or in experimenting.

The basic steps are:

(1) The fibrous rayon starting material (preferably in continuous web, yarn, roving or tow form) if not received in a scoured or naturally clean condition, is scoured to remove sizings or other contaminants; suitable procedures for cleaning being well known in the textile in dustry. For instance, starch sizings on warp yarns can be removed by washing with soap or detergent solution, or by enzymatic action; and oil lubricants with a volatile solvent such as mineral spirits. A final scouring to remove residual contaminants may be desirable, such as with ammonia water (e.g., 28% ammonium hydroxide solution) or with 4% sodium hypochlorite solution. The scoured fabric may be dried at about 125 F. The clean dry rayon starting material is impregnated throughout with an aqueous solution of suitable water-soluble monobasic metal phosphate salt such that the saturated fibrous material, after squeezing to a damp state to remove excess liquid and to ensure that all fibers are treated, contains the salt in a sufiicient proportion to render the fibers, even when dried, nonfiammable and able to undergo the subsequent thermochemical transformation to provide product fibers containing at least 3% by weight of combined phosphorus. A hot salt solution concentration of about to 30% gives best results. As previously stated, the metal phosphate salts which we employ are monomagnesium phosphate, monocalcium phosphate, monosodium phosphate, monopotassium phosphate, and mixtures thereof.

These metal phosphate salts also result in at least about 3% by weight of magnesium, calcium, sodium or potassium being incorporated in the polymeric molecules of the product fibers. Boric acid (or equivalent boron compound) may also be included so as to result in product fibers of higher tenacity that include at least 0.5% by weight of carbon-bonded boron atoms which enhance fiber stability at high temperatures. A preferred hot salt solution contains of the metal phosphate salt and 4% boric acid. A pickup of salt solids (dry basis by weight) of about 10 to 30% appears to give best results. Immersion for about 5 minutes in a near boiling salt solution is preferably employed to facilitate impregnation of the fibers, excess solution then being removed to give about a 100% wet pickup of salt solution.

(2) The damp salt-treated fibrous material is then heated until dry, preferably at about 140 to 250 F. The dry rayon fibers are thus impregnated (and not merely coated) with the salt owing to the permeability of the fiber structure to such aqueous salt solutions. If this treatment has caused bundling and sticking together of fibers, further processing will be improved by flexing, rubbing, or tumbling the salt-impregnated material to loosen and free the individual fibers so they will be fully exposed to the atmosphere during heating.

(3) The dry salt-impregnated rayon fibers are next heated in an oven at an elevated temperature and for a time which is short but is sufficient to bring about the desired transformation of the fibers to the previously described strong and flexible phosphorus-containing black state. Air is not excluded. On the contrary, heating in the presence of air (or equivalent oxygen-containing atmosphere) appears to be essential to the production of the presently desired product fibers. Convenient use can be made of a vertical tower-type of air-circulating oven for continuous manufacture. The preferred general temperature range is about 450 to 600 F. When a plain air atmosphere is employed. If the temperature is too high for the existing conditions and period of exposure, an uncontrollable exothermic reaction (exotherm) develops and the fibers or fabric will glow red and yield a brittle product. The optimum temperature-time combination can be determined by trial-and-error under any given set of manufacturing conditions. The air flow should be regulated to avoid a destructive exothermic reaction (exotherm); a decrease in the rate at which air enters the oven resulting in a lower concentration of oxygen in contact with the hot fibers due to the diluting action of decomposition vapors emitted from the fibers. Optimum temperatures when a circulating air oven is employed are usually in the range of about 500 to 550 F., and corresponding optimum times in the inverse range of about 30 to 5 minutes. The most desirable heating period is one that is long enough to develop the maximum tensile strength, beyond which point the tensile strength starts to diminish. The treatment of light rayon fabrics appears to be optimized at a combination of about 550 F. and five minutes. In the case of heavy close-woven cloths (such as one weighing about 20 oz. per square yard), it is desirable to conduct the heat treatment in successive stages at increasing temperatures to avoid a too-rapid temperature rise in the interior of the fabric (resulting from the exothermic nature of the reaction and the poor heat conductivity of the fibers), as by heating for successive ten minute periods at approximately 475 F., 500 F. and 525 F. All of these temperatures refer to the temperature of the air in proximity to the fibrous material, which enters the oven at a much lower temperature and builds up in temperature in response both to the external heating and to the internal heating produced by the exothermic chemical reaction in the fibers.

A further expedient for effectively regulating conditions so as to avoid an uncontrolled or excessive (run away) exothermic reaction is to employ an oven atmosphere of lower oxygen content than plain air (which contains 23% oxygen by weight). Thus use can be made of air diluted with steam, nitrogen or ammonia so as to have an oxygen content which is substantially less than 23% and which may be as low as about 5% by weight or even less in some instances. Or use can be made of an equivalent artificial air atmosphere comprised of a mixture of oxygen with nitrogen, steam or ammonia. These expedients permit of using higher oven temperatures (up to about 900 F.) and correspondingly shorter heating periods (down to one minute or even less). Air diluted with nitrogen or ammonia to an oxygen content of approximately 5 to 10% by weight is a preferred example for use at oven temperatures of about 550 to 700 F. or somewhat higher and exposure periods of about two to five minutes, and conversion can be followed by cooling in air to room temperature. It is also possible to use an equivalent two-stage oven process in which the salt-impregnated fibrous rayon material is converted to a black phosphorus-containing state in an oxygen-free atmosphere (e.g., a steam, nitrogen or ammonia atmosphere), at a temperature which may be as high as about 900 F., and is then further heated in air or other oxygen-containing atmosphere to complete the conversion to the desired final state (apparently by freeing the fibers of an easily-oxidizable decomposition product); resulting in lustrous black fibers free from by-product contaminants.

Another expedient is to employ a short conversion period (not over about 5 minutes), at a temperature in the range of 600 to 900 F., with conditions adjusted so that the hot fibers only contact an atmosphere of low oxygen content; the hot converted material being then immediately exposed to a non-oxidizing atmosphere (such as pure nitrogen) to minimize harmful oxidation action. (This procedure can be used in a multistage continuous process wherein the insulative black fibrous product is promptly carbonized at a high temperature without intermediate cooling.)

Salt impregnated rayon yarn strands can be continuously transformed by being pulled through a heated tube through which nitrogen flows counter-currently (being introduced under pressure at the exit end of the tube which may, for instance, have a length of 3 feet). This permits of close control to prevent an exotherm and obtain a product of maximum strength. In this case the yarns bring some entrapped or occluded air with them into the tube and thus are exposed to a hot oxygen-containing atmosphere during pyrolysis, or initial pyrolysis, followed by exposure to an oxygen-free nitrogen atmosphere.

(The term air when used hereafter in a broad generical sense embraces not only natural air but other useful oxygen-containing atmospheres.)

(4) The black insulative phosphorous-containing fibrous product, after cooling, may be washed to remove soluble materials that are present (and which might adversely affect desired properties of the product for particular usages). This washed and dried end product, prepared under substantially optimum conditions, has a weight which is approximately two-thirds that of the corresponding original rayon fibers (dry basis). Shrinkage of a woven colth is about 10%. The fibers are microporous and are hygroscopic as illustrated by the fact that they can absorb up to about 20% by weight of water when exposed to an atmosphere of 100% relative humidity at 72 F. Fabrics and fibers which have been conditioned by exposure to a humid atmosphere are more flexible and stronger than those that are extremely dry.

These insulative products (washed or not) are suitable for sale and for many uses without further thermal or chemical treatment. They have good strength and flexibility, the tensile strength being at least 20% (and preferably at least about 40%) that of the original rayon starting material. The corresponding tenacity-retention percentages are higher since tenacity values in grams per denier involve the fiber weight per unit length, and the black fibers weigh less than the original rayon fibers from which they are made. Tenacity values of fibers and yarns of 0.4 gram per denier and higher can be obtained. Woven products having a tensile strength of at least 20 lbs. per inch width can be made.

Analyses of various washed and dried sample products prepared under various conditions indicate that these elements can be present in proportions within approximately Boron in proportions from 0.5% up to about 5% is also present when boric acid is included in the salt impregnant. The balance in each case is oxygen (in the range of about 30 to 40%). In comparison, pure cellulose (dry basis) has a composition of 44.4% carbon, 6.2% hydrogen and 49.4% oxygen. The washed and dried end products commonly weigh 60 to 67% as much as the rayon starting material from which made. These facts demonstrate the radical chemical transformation undergone by the cellulose molecules; and also the continued organic nature of the fibers. The original crystalline structure of the rayon cellulose fibers is lost, the black fibers being amorphous.

EXAMPLE 1 The rayon fabric employed in these experiments was a commercial cloth, weighing 8 ounces per square yard, woven in a plain weave from viscose-rayon staple yarns comprised of 1.5 denier fibers. Samples were impregnated with each of the aforesaid monobasic magnesium, calcium, sodium and potassium phosphate salts and converted by heating to the insulative black state. The product samples were examined and tested, and were employed to make up samples of an arc-proofing insulating tape which were tested and found satisfactory in each case.

In each instance a fabric sample of inch width and 54 inch length was scoured until free of sizing (using a 28% ammonium hydroxide solution) and dried. It was dipped in a solution in hot distilled water of the particular metal phosphate salt being studied. Excess solution was removed from the saturated wet cloth by patting with a clean dry cloth and it was hung up to dry. The dry cloth was softened by tumbling or rubbing and was then hung in free loops from the cover of a box-like sheet aluminum receptable located in an oven and equipped with inlet and outlet means for enabling a desired air or gas mixture to be passed through the interior to expose the fabric to a controlled atmosphere. The cover was removed from the oven to permit suspending the fabric from it. In these experiments a nitrogen-oxygen mixture in 90:10 ratio was employed as the air and the flow rate was such that half the atmosphere in the box was replaced every 15 seconds. The gas mixture entered the box through a coiled copper tube which was sufficiently exposed in the oven to preheat the nitrogen-oxygen gas to oven temperature. The atmosphere in the box also included gases evolving from the salt-impregnated fabric during pyrolysis. The temperature of the ambient atmosphere in the box was measured by a thermocouple located next to the fabric. The oven was regulated so as to maintain a temperature of approximately 550 F.

At the end of an approximately ten minute exposure at 550 F., the cover of the box with its suspended black sample of converted fabric was removed from the oven and, after cooling, the fabric was dipped in 0.05 N sodium hydroxide neutralizing solution, thoroughly rinsed in distilled water and dried.

Elemental analyses showed the following compositions for fabrics converted from the rayon (cellulose) starting fabrics impregnated with the respective metal salts listed in the first column. The metal content in each case is the alkali or alkaline earth metal of the particular salt (magnesium, calcium, sodium or potassium). All percentages are by weight. Oxygen constituted substantially all of the remainder and was thus present within the calculated range of approximately 32 to 37% in all cases.

Element (percent b y Analysis for nitrogen failed to show any definite indication beyond the limit of experimental uncertainty, indicating that under the thermal processing conditions employed there was no fixation of nitrogen from the nitrogencontaining ambient atmosphere to which the fabric was exposed during conversion.

In each case the lustrous black woven fabric product had a weight equal to about two-thirds that of the corresponding original rayon cloth; showed a shrinkage of about 10%; was highly flexible; had a breaking strength at least 20% of that of the original rayon cloth and was thus usefully strong and flexible; was non-flammable and was resistant to prolonged heating in air such as to permit of use in place of asbestos fabric; and was electrically and thermally insulative. Use of the magnesium and calcium salts resulted in the strongest products.

A valuable industrial use of the type of black insulative woven fabric described above is as a substitute for asbestos fabric in electrical arc-proofing and fire-proofing tape construction where the fabric carries a heavy coating on one side of a flame retardant elastomer (such as polychloroprene or a plasticized polyvinyl chloride). This tape is wrapped upon high voltage power cables in manholes, cable trays, switohboxes and substations, etc. Samples were made using each of the different black fabric product types described above. The fabric was coated with a polyvinyl chloride plastisol in a coating weight of 40 to ounces per square yard, heated for 4 to 5 minutes at 350 F. to fuse the coating, and slit into tape. All were found to be suitable for this usage.

Conductive carbonized fiber products An important use of the aforesaid black phosphorusand-metal-containing organic nonconductive fiber materials of this invention (which contain about to carbon) is in making partially carbonized to highly carbonized electrically conductive fiber materials in which fiber identity is retained and fiber tenacities of at least 0.3 (and up to 2 or more) grams per denier can be obtained with adequate flexibility. Woven fabrics, knitted fabrics, braided fabrics, nonwoven fabrics, yarns, rovings,

and continuous filament tows can all be converted by continuous procedures Without loss of fiber identity. The fibers are smooth, lustrous and black.

This conversion can be effected by rapid heating (not over 30 minutes being needed and less than minutes being usual) in a nonoxidizing environment at a temperature in the range of about 500 to 2600 C. (about 950 to 4700 F.) or higher. The higher the final peak temperature the higher the carbon content and the higher the electrical conductivity. Examples of such nonoxidizing ambient atmospheres are nitrogen, steam, hydrogen, molten metals and a vacuum. The function atmosphere in contact with the hot fibers will include non-oxidizing vapors emitted from the fibers during the pyrolytic conversion. The volatile products are largely evolved during the first seconds and include water, carbon dioxide, carbon monoxide and decomposition products of the salts. Carbonization can also be effected by envelopment in a nonoxidizing flame.

Use of a nitrogen atmosphere is preferred not only for practical and economic reasons but also because nitrogen fixation can occur when a temperature of about 2500 F. (about 1400 C.) or higher is employed, which incorporates carbon-bonded nitrogen atoms in the molecular structure of the fibers, thereby increasing the thermal stability of the resultant fibers and improving the carbonization as regards strength and flexibility of the product fibers.

These carbonized fibers may contain carbon in proportions ranging from about 70% to higher than 99% (i.e., to 99+% and can range in electrical conductivity from semiconductors to good conductors (the fiber resistivities ranging in order of magnitude from 10- to 10 ohmcm.). Strong flexible woven black fabrics can be produced having carbon percentages ranging from 70 to 90% and fiber resistivities ranging from 10 to 0.1 ohm-cm. (the voltage/current ratio being measured by using the wellknown four probe method). Woven carbon fabrics, carbonized to a final temperature of 1400 C. (2550 F.) or higher, can be formed with carbon contents of 70% or higher and good electrical conductivities. The fibers are microporous and hygroscopic as indicated by substantial moisture regain values when the fabric is dried, weighed, exposed at room temperature to air of 50% or higher relative humidity, and reweighed. Highly flexible and stretchable knitted carbon fabrics can be made. Depending upon the treatment, the temperature coeflicient of resistivity may be positive or negative. In general, the higher the carbon content the smaller the temperature coefficient.

Fibers and fabrics containing from about 70% up to 90% carbon may be referred to as partially carbonized, and those containing above 90% carbon may be referred to as carbon fibers and fabrics. All such fibers and fabrics. All such fibers and fabrics may be designated as conductive carbonaceous fibers or fabrics.

These fibers are predominately amorphous and even those of highest carbon content (99+% carbon) are not graphitized carbon, and are not graphite carbon fibers per se, although some degree of polycrystalline graphitic type structure may be present as shown by X-ray diffraction patterns.

These carbonized fibers and fabrics, owing to the nature of the novel black phosphorus containing and metal-containing organic fibrous material that is subjected to the carbonizing process and to the retention in appreciable proportions of combined elements in the carbon structure during at least part of the carbonizing process, and also owing to the rapidity of the process, are significantly different than those reported by other workers which are made by lengthy heating of cellulose fiber material up to high temperatures reached at a slow rate or by gradual increments, requiring carefully controlled heating over a period of several hours or more. (Thus see British Spec. No. 965,622 published Aug. 6, 1964.) They are also different from the partially carbonized fibers which have been made by fairly rapid pyrolysis of cotton and other cellulosic fibers by successive heat treatments at temperatures ranging from about 300 F. to above 1500 F., requiring the substantial absence of oxygen during all of the heat treatments. (Thus see US. Patent No. 3,001,987 issued Dec. 5, 1961.) Carbonized cellulose fibers and fabrics have been graphitized to a highly graphitic carbon state by prolonged heating in an oxygen-free atmosphere up to a final temperature which has to be of the order of 2700" C. or higher, preferably at least 3000 C. (Thus see US. Patent No. 3,107,152 issued Oct. 15, 1963, and the article by Schmidt and Jones in Chemical Engineering Progress, issue of October 1962, at pp. 42-50.)

Continuous factory or laboratory production of uniform material can be achieved by passing the earlier-described non-conductive type of black organic fabric, yarn, roving or tow through a horizontal furnace having electrical radiant heating elements located above and below the moving material, the latter efficiently absorbing the radiant energy because of its blackness. Air is flushed out by introduction of a nonoxidizing gas such as nitrogen. During operation of the process, pyrolysis results in nonoxidizing vapors being emitted from the hot fibers and providing a nonoxidizing furnace atmosphere. This can be augmented by addition of nitrogen if necessary or desirable to maintain a nonoxidizing environment. Successive heating in two or more furnace zones may be employed, each zone being provided by a successive furnace section equipped to provide a higher temperature level. Conveniently, each furnace section can be about two feet in length when feed rates of 1 to 10 feet per minute are used. Nitrogen or other nonoxidizing gas is introduced, and the exhaust is regulated, such as to maintain a nonoxidizing atmosphere in each furnace section. A minimum of tension should be used. Nonwoven staple fiber fabrics or batts which are weak can be transported on a carrier web. Temperatures are measured with thermocouples located close to the travelling fibrous material or by an optical pyrometer. The longer furnace exposure (at the slower travel rates) give a lower weight yield but the fibrous material is stronger and more flexible. Products of highest strength result when the final temperature is at least about 1400 C. (2550 F.).

The carbon content and specific properties of any given carbonized product depend not only on the final furnace temperature, and exposure timing, but on the nature and composition of the particular nonconductive black organic precusor fibrous starting material that is converted. Black precursor fibers containing 0.5 to 2.5% by Weight of combined boron (presumably carbon-bonded boron atoms diffused through the carbon structure) are more resistant to carbonizing temperatures and the conductive product fibers are stronger and more flexible than those made without use of boric acid (or equivalent) as a component of the salt impregnant of the rayon fiber starting material.

EXAMPLE 2 Samples of the four types of insulative black woven fabrics produced as described in the preceding example (using monobasic magnesium, calcium, sodium and potassium phosphate salts, respectively, as the rayon fabric impregnants) were carbonized to a black electrically conductive state by exposure in a furnace heated to approximately 1400 C. (approximately 2550 1 the residence time being three minutes. The furnace was flushed with pure nitrogen and nitrogen was introduced during opera tion so as to maintain a nonoxidizing atmosphere.

Despite shrinkage and loss of weight the structural fiber identity was maintained and a flexible black conductive cloth was obtained in each case, having a warp direction breaking strength in the range of 4 to 10 lbs. per inch width and a thickness of about 25 mils (0.025 inch). The fibers were found to have specific electrical resistivities of approximately 0.1 ohm-cm. The resistance of the fabric in each case was approximately 2 to 4 ohms per square. Con-,

Element (percent by weight) Salt metal H Metal Magnesium 82 0. 0 3. 2 2 Calcium 74 1. 1 6. 2 8 Sodium 75 1. 1 4. 5 5 Potassium 70 1. 2 4. 5 8

Analyses for combined nitrogen indicated in each case the probable presence of approximately 0.1 to 0.3% by weight of nitrogen atoms, presumably coming from the nitrogen atmosphere in the furnace and combining with carbon atoms of the fibers during pyrolysis at the high temperature involved. (Conclusive evidence of nitrogen fixation was obtained in the work hereafter described in Example 3.)

It will be noted that these conductive carbonized fibers of 70 to 82% carbon content contain substantial proportions both of phosphorus and of the metal element of the salt impreguant; these not having been eliminated during high temperature pyrolysis. The presence of atoms of these elements in combination with the carbon atoms of the fiber structure undoubtedly has an importance influence upon the carbonizing transformation and upon the physical state and properties of the carbonized fibers. Rapid carbonization is made possible in obtaining products of useful flexibility and strength and having good thermal stability.

EXAMPLE 3 This example illustrates a process for rapidly and continuously converting rayon yarns (impregnated with metal phosphate salt) to conductive black fiber yarns of good flexibility and strength. The yarn is passed through a multi-stage furnace so as to be converted from rayon yarn to the insulative black yarn (50 to 65% carbon) and then be immediately carbonized at a higher temperature in a nitrogen atmosphere to a carbon content of at least about 80%. This example illustrates the use of a substantial minor proportion of boric acid as a salt mixture component (mixed with a major proportion of the metal phosphate salt, e.g., the magnesium or sodium salt). This expedient introduces boron atoms into the fiber structure (in addition to the phosphorus and metal atoms) and causes the carbonized product yarn to be much stronger than would otherwise be the case. Fiber tenacity values of at least 2 grams per denier can be obtained. The product yarn also contains combined nitrogen derived from the oxygen-free nitrogen atmosphere of the carbonizing furnace.

The three-stage furnace consisted of a horizontal three-zone externally electrically heated tube furnace (internal diameter of one inch). There was no spacing between the first two zones (each 18 inches long and comprising a single glass tube 36 inches long, heated in two zones). A space of 2 inches was provided between the second and third zones (the latter being 18 inches long and comprising a ceramic tube). The third zone provided the carbonizing stage. Means were provided for pulling the yarn through the furnace under minimum tension and winding up. Pure nitrogen from a pressure tank was introduced into the exit end of the third stage so as to fiow counter-currently to the advancing yarn at a rate of 6 cubic feet per hour. The atmosphere in the first two zones, during operation, was provided by air entering the glass tube with the yarn and which was diluted by the nonoxidizing vapor mixture emitted from the salt-impregnated yarn during pyrolysis in the furnace. Gases escaped from both directions into the 2 inch space separating the second and third zones and were removed by a forced draft ventilating hood.

The starting yarn was in a clean state as received and hence needed no scouring. It was a 3.5 Z-twist per inch yam prepared from a 1650 denier continuous filament high tenacity viscose-rayon yarn. This yarn was continuously dipped into a salt solution bath, dried over an electric heat lamp, and drawn into and through the aforesaid furnace.

Magnesium salt experiment.-The hot salt bath was a solution in deionized water of 12% monomagnesium phosphate salt and 4% boric acid, by weight. The pull rate of the black yarn product leaving the furnace was 6 inches per minute. The first two zones were operated at 650 F. and the third zone (carbonizing stage) at 2600 F.

The black conductive yarn product had a breaking strength of 610 grams, a denier value of 10-80, a tenacity of 0.56 gram per denier, and an electrical resistivity of 18 ohms per inch.

Samples were extracted with deionized water for 24 hours to remove any soluble components and were then analyzed; showing the presence of the following elements in percentages by weight:

Carbon 79.5 Hydrogen 0.7 Nitrogen 4.5 Boron 2.5 Magnesium 2.5

In a control experiment, argon was used in place of nitrogen in the third stage. Nitrogen analyzed at 0.1% in the product in this case (which value is of the same order as the probable analytical error).

In a comparable experiment in which boric acid was omitted, the boron-free yarn product was about half as strong.

Sodium salt experiment-11: this case the hot salt bath contained 20% monosodium phosphate salt and 4% boric acid. The pull rate was 12 inches per minute. The first zone was operated at 490 F., the second zone at 620 P., and the third (carbonizing) zone at 2600 F.

The black conductive yarn product had a breaking strength of 2010 grams, a denier value of 775, a tenacity of 2.6 grams per denier, and an electrical resistivity of 21 ohms per inch.

Analysis showed the following (percent by weight):

The breaking strength was several times greater than could be obtained in experiments in which the boric acid was omitted and the sodium phosphate used alone.

It will be noted that the breaking strength and tenacity values were much higher when the sodium salt was used rather than the magnesium salt (both being used in combination with boric acid). However, the yield of yarn on a denier basis was substantially less.

The analyses given above do not include phosphorus values but from other work it is known that combined phosphorus is present in fibers of this type.

In a similar experiment in which the first stage was operated at 450 F., and the second stage at 530 F. (the third stage being at 2600 F. as before), the black yarn product had a breaking strength of 2540 grams, a denier value of 835, a tenacity of 3.0 grams per denier and a resistance of 30 ohms per inch.

EXAMPLE 4 In this experiment, samples of black conductive yarns of the preceding example (made from rayon yarns impregnated with the magnesium or sodium salt and boric acid mixture) were further carbonized to provide useful flexible carbon fiber yarns of approximately 99% carbon content. The two samples (each about 100 yards long) were wound onto a carbon rod which was placed in a carbon boat that was introduced into a carbon tube electric furnace. The furnace was continuously purged with pure nitrogen during the experiment. A temperature of 2500 C. (about 4500 F.) was reached in about seven hours at which time the power was turned off. The furnace was cooled to room temperature overnight after which the samples were removed. (This lengthy heating period is not required but was employed in this experiment owing to the slow rate at which the particular furnace could be heated up.)

The carbon yarn products had the following measured properties, column (A) representing the yarn derived from the rayon yarn that had been impregnated with the magnesium salt and boric acid, and column (B) the yarn derived from rayon yarn impregnated with the sodium salt and boric acid:

These carbon fibers retain a trace of combined nitrogen in the structure and a low order of graphitic carbon development. Fibers of this type are suitable for use as highly stable power resistors, and for use in highly stable conductive papers to be employed as resistors and as electrical heating elements that will not greatly vary in resistance with prolonged use and at different temperatures. Cellulosic papers containing about to 50% by Weight of dispersed staple-length carbon fibers (chopped from these yarns) can be readily and economically produced.

The various types of carbonized fibers, which are smooth but have micropores, can be modified, whether in fabric or free fiber form, by providing a thin film, coating or deposit which can be strongly bonded by virtue of the microporous fiber surface structure, despite chemical inertness of the fiber. Thus a deposit of pyrolytic graphite, a carbide formation, an oxide coating, a silica or silicate coating, a metallic film or coating, a coating of a high-temperature-resistant polymer (which may be polymerized in situ), a sizing or priming coating to enhance bonding to a further coating or to a varnish or resin in which embedded, are illustrations.

Further uses of conductive fiber products As already indicated, the carbonized products disclosed above have many and unique fields of utility owing to the variety of combinations and ranges of properties made available.

Carbon fiber products of over 90% carbon content combine good electrical conductivity with excellent thermal stability, including stability to lengthy exposure to high temperatures even in the presence of air. Thus woven and knitted fabrics can be used as flexible and conformable electrical heating elements for laboratory ware. An advantage over the use of metal heating wires embodied in a heat-resistant fabric of some sort is that such wires need to be of fine gauge and are relatively fragile, and such fabric constructions are not as flexible and conformable as those made possible by use of conductive carbon fibers.

The various types of conductive fibers that can be produced provide for a wide choice of properties in making up papers, nonwoven fabrics, films, coatings and laminates, containing such fibers in various proportions to provide desired electrical and other properties.

These carbon fabrics and fibers can be combined with high-temperature-resistant resins to provide dense voidfree molded or compressed conductive solid articles and laminates of great thermal stability. For example, a carbonized woven cloth of 98% carbon content or higher can be impregnated with a thermosetting phenolic laminating varnish which is commercially available and specifically designed and sold for high temperature applications (e.g., Resinox SC-lOOS phenolic resin varnish sold by Monsanto Chemical Co.) so as to have a resin pickup of 35% when dried. Sufficient layers of this pre-impregnated cloth are laminated together to form a composite one inch thick. This is then compressed for one hour under a pressure of 1000 p.s.i. at a temperature of 350 F. The resultant cured sheet is /2 inch thick and is hard, dense and rigid and had a high electrical conductivity and a relatively low temperature coefiicient of resistivity. These properties suggest electrical use for making potentiometer elements, etc. This sheeting had a relatively low thermal conductivity, low thermoexpansibility, high thermal shock strength. This type of product has low erosion and ablation rates even at plasma jet temperatures (of the order of 18,000" E).

Molded parts can be made by charging the mold with chopped fabric or chopped tow fibers pre-impregnated with a thermoset resin such as a high-temperature-resist: ant phenolic or modified phenolic resin. The bulk charge and the mold can be preheated to enhance the fiow in the mold.

Ablative and other characteristics can be modified by incorporating inorganic fillers, such as metal oxides (e.g., zirconium dioxide or other refractory oxide), metal powders and flakes, etc., in the varnish or resin impregnant. Properties can also be modified by inclusion of fibers of other types such as graphite fibers, glass fibers, silica fibers, metal fibers, aluminum oxide fibers, aluminum silicate fibers, fluoro-carbon fibers, etc.

Liquid epoxy molding and coating compositions containing dispersed carbon fibers can be provided which are capable of being cured in situ to provide either flexible or hard conductive molded parts and coatings.

Woven fabrics, nonwoven fabrics, and carbon wool can be used in unusual and demanding filter and adsorption applications which capitalize on their chemical inertness, adsorptive properties, or thermal properties, or combinations thereof.

Pressure-sensitive adhesive coating compositions containing dispersed conductive carbonized fibers can be coated out to form, upon drying, a normally and aggressively tacky, viscoelastic, fiber-reinforced conductive adhesive coating upon either a permanent base or backing, or a temporary removable support having a release surface to permit of transferring the adhesive layer to another surface. Electical conductivities over a wide range are possible by selection of proportions and type of fiber. Carbonized continuous filament strands can be embedded in lineally aligned fashion in pressure-sensitive adhesive tape structures (for example in the viscoelastic adhesive layer) to provide a desired degree of electrical conductivity. Thus lineal conductivity through the carbonized filaments can be provided in a tape which is otherwise insulative.

Adhesive tapes having electrically conducting backings can be produced by using conductive carbonized woven, knitted, braided or nonwoven fabric backings which may or may not be impregnated or coated with rubbers, varnishes, etc. Knitted fabrics permit of tapes which are stretchable and conformable in both directions. Conductive papers containing carbon fibers can also be used to provide conductive paper-backed adhesive tapes. Hightemperature-resistant pressure-sensitive adhesive tapes can be made using high-temperature-resistant pressuresensitive adhesive, such as certain silicone adhesives.

We claim:

1. A process of thermochemically converting regenerated-cellulose fiber starting material to corresponding black insulative organic fiber material, which comprises impregnating clean starting material with an aqueous solution of a salt composition consisting essentially of at least one metal phosphate salt of the class consisting of monomagnesium phosphate, monocalcium phosphate, monosodium phosphate and monopotassium phosphate, the pickup of salt being about 10 to of the fiber Weight, drying, and heating the dry salt-impregnated fiber material for a short time and in the presence of air at a temperature of at least about 450 F., the conditions being controlled so as to avoid a destructive exotherm and such as to result in a flexible black insulative fiber material having a fiber carbon content in the range of about 50 to 65%, a phosphorus content of at least about 3%, and a content of at least about 3% metal derived from the metal phosphate salts in the impregnant composition, the heating process being terminated before degradation to weak brittle fibers occurs.

2. A process according to claim 1 wherein said salt composition includes a substantial minor proportion of boric acid which imparts at least 0.5% boron to the composition of the product fibers.

3. A process according to claim 1 wherein said salt composition consists essentially of a major proportion of monomagnesium phosphate and a substantial minor proportion of boric acid.

4. A process according to claim 1 wherein said salt composition consists essentially of a major proportion of monosodium phosphate and a substantial minor proportion of boric acid.

5. New and useful black organic fibrous material made by the process of claim 1.

6. Black insulative woven fabrics made from corresponding woven rayon fabrics by the process of claim 1.

7. New and useful black organic fibrous material made by the process of claim 2.

8. New and useful black organic fibrous material made by the process of claim 3.

9. New and useful black organic fibrous material made by the process of claim 4.

10. A process of thermochemically converting regenerated-cellulose fiber starting material to corresponding black insulative phosphorus-containing organic fiber material and then carbonizing the latter to provide corresponding conductive fiber material, which comprises impregnating clean starting material with an aqueous solution of a salt composition consisting essentially of at least one metal phosphate salt of the class consisting of monomagnesium phosphate, monocalcium phosphate, monosodium phosphate and monopotassium phosphate, capable of imparting to the insulative product fibers a phosphorus content of at least 3%, drying, heating the dry salt-impregnated fiber material for a short time and in the presence of air at a temperature in the range of about 450 to 900 F., the conditions being controlled so as to produce flexible black insulative fiber material having a carbon content in the range of about 50 to 65% and a phosphorus content of at least 3%, and subsequently carbonizing this fiber material by rapid heating in a non-oxidizing environment to a final temperature of at least 950 F. to produce electrically conductive fibers having a carbon content of at least 70%.

11. A process according to claim 10 wherein said salt composition includes a boron compound adapted to impart at least 0.5% boron to the composition of the insulative organic fiber material.

12. New and useful electrically conductive carbonaceous fiber material made by the process of claim 10.

13. New and useful electrically conductive carbon fiber material having a carbon content above 90%, made by the process of claim 10.

14. New and useful electrically conductive fiber material containing at least carbon, made by the process of claim 11.

15. New and useful electrically conductive fiber material containing at least 70% carbon and at least about 2.5% boron, made by the process of claim 11.

16. A process of thermochemically converting regenerated-cellulose fiber starting material to corresponding black insulative phosphorus-containing organic fiber material and then carbonizing the latter to provide corresponding conductive fiber material, which comprises impregnating clean starting material with an aqueous solution of a salt composition consisting essentially of a major proportion of at least one metal phosphate salt of the class consisting of monomagnesium phosphate, monocalcium phosphate, monosodium phosphate and monopotassium phosphate, and a substantial minor proportion of boric acid, drying, heating the dry salt-impregnated fiber material for a short time in the presence of air at a temperature in the range of about 450 to 900 F. to produce a flexible black phosphorus-containing organic fiber material, and subsequently carbonizing by rapid heating in a nonoxidizing nitrogen atmosphere to a final temperature of at least 2500 F. to produce flexible electrically conductive fibers having a carbon content of at least 17. New and useful electrically conductive fiber material containing at least 80% carbon made by the process of claim 16.

18. A process according to claim 16 wherein said salt composition consists essentially of a major proportion or" monosodium phosphate and a substantial minor proportion of boric acid.

19. New and useful electrically conductive fiber material containing at least 80% carbon, made by the process of claim 18 in such manner that the fiber tenacity value is at least 2 grams per denier.

20. A process according to claim 16 wherein the final carbonizing temperature is at least about 4500 F. and processing conditions are controlled to yield useful flexible carbon fibers of approximately 99% carbon content.

References Cited UNITED STATES PATENTS 9/1962 Abbott 252502 XR 2/1966 Peter-s 8ll6 US. Cl. X.R. 

